The present invention relates to the field of casting, and more particularly to producing ceramic cores for casting, which cores are to be received in casting molds for forming internal cavities in castings made in such molds.
Such cores may be used in particular in lost model or lost wax casting methods. So-called “lost model” or “lost wax” casting methods have been known since antiquity. They are particularly adapted to producing metal parts of complex shapes. Thus, lost wax casting is used in particular for producing turbomachine blades. In the present context, the term “turbomachine” is used to designate any machine in which it is possible to transfer energy between a flow of fluid and at least one rotor, such as for example a compressor, a pump, a turbine, a propeller, or a combination of a plurality of such elements. The term “blade” is used to cover any airfoil element, whether stationary or rotary, that contributes to this transfer of energy in the turbomachine. Lost wax casting is particularly useful for production.
In lost wax casting, the first step is normally to make a model out of a meltable material, having a melting temperature that is comparatively low, such as for example wax or resin. The model is then integrated with a casting tree that is to be covered in refractory material in order to form a mold. After the meltable material of the mold has been removed or eliminated from the inside of the mold, whence the term “lost” in the name for such methods, molten metal is cast into the mold in order to fill the cavity in the mold that is formed by the model being removed or eliminated from the mold. Once the metal cools and solidifies, the mold can be opened or destroyed in order to recover a metal part having the shape of the model. The term “metal” is used in the present context to cover not only pure metals, but above all metal alloys.
In order of form complex internal cavities within metal parts that are obtained by such casting methods, it is possible to integrate one or more refractory cores in each model. These refractory cores remain in position inside each mold after the meltable material has been removed, thereby enabling complex shapes to be formed inside the casting that is made in the mold. They can subsequently be eliminated together with the rest of the mold when unmolding the part.
Typically, such refractory cores are made of ceramic material and constitute consumables in the casting method. To produce them, injection-molding methods have been developed in which a paste comprising a ceramic granulate and a polymer binder is injected under pressure into a mold cavity in order to form the core, with the core then being, fired in order to consolidate it. In order to further increase the ability of the core to withstand the forces to which it is to be subjected during the casting process, the core may also be impregnated with a resin after its initial firing, prior to being, fired again a second time.
Nevertheless, the reject rate when producing such cores can be relatively high, particularly when they are complex in shape. This is due to internal stresses that are generated during the firing and subsequent cooling of the core, which internal stresses can give rise to cracking in certain critical points of the core. In particular, the core may present lateral protuberances, especially when it is to form a cooling circuit in a turbomachine blade, which lateral protuberances may then be intended to form outlet channels leading to an outside surface of the blade, and in particular outlet slots in the trailing edge of the blade. Under such circumstances, cracks can form at the root of at least one of the lateral protuberances.